Currently, as media on which a large amount of data is recorded, optical recording media, such as a CD (compact disk) and a DVD (digital versatile disk), are widely used. In recent years, there is a demand for recording a high-definition moving image and a demand for recording a larger amount of data due to the development of personal computers. In optical recording media, such as a CD or a DVD, a sufficiently high density cannot be achieved for recording such a large amount of data, as a result of which a large amount of data cannot be recorded on one disk. Therefore, for recording such a large amount of data, recording must be performed while replacing a plurality of disks. In recent years, a hologram recording medium on which a very large amount of data can be recorded compared to, for example, currently used CDs and DVDs is drawing attention.
Hologram information recording is a method in which a light beam is separated into two coherent light beams, data modulation is performed on one of the light beams by a spatial light modulator to form a signal light, and the other light beam is combined as reference light at a recording medium, so that data is recorded as an interference pattern. Then, by irradiating the recording medium with reference light that is the same as that used during the recording, the signal light can be detected as reproduction light, so that the data can be reproduced. In this hologram information recording/reproducing operation, when the light wavelength and irradiation angle during the reproduction of the information are different from the light wavelength and irradiation angle during the recording of the information, the signal light is not reproduced. By making use of this characteristic, the light wavelength and the irradiation angle during the recording/reproduction of the information are changed to perform multiplexing of data on the same area of the recording medium, as a result of which the data is recorded.